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Investment securities - Narrative (Details) | 12 Months Ended
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Dec. 31, 2020 USD ($) security | Dec. 31, 2019 USD ($) security | Dec. 31, 2018 USD ($)
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Accrued interest receivable | $ 43,603,000 | $ 17,083,000 |
Marketable securities at fair value | 4,591,000 | 3,295,000 |
Trade date payable - securities | 0 | 0 | $ 2,120,000
Net unrealized gains on equity securities | $ 296,000 | $ 148,000 |
Number of securities in securities portfolio | security | 514 | 365 |
Number of securities in securities portfolio, unrealized loss position | security | 16 | 58 |
Allowance for credit losses for investments | $ 0 |  |
Provision for credit losses on available for sale debt securities | 0 |  |
Other than temporary impairment of equity securities |  | $ 0 |
Debt Securities |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Accrued interest receivable | 4,540,000 | 2,843,000 |
Collateral Pledged |  |  |
Debt and Equity Securities, FV-NI [Line Items] |  |  |
Securities pledged | $ 804,821,000 | $ 373,674,000 |

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